The vectorial active transport of solutes and water across epithelial membranes results from the different morphological, biochemical and functional properties of the apical and basolateral aspects of the plasma cell membrane. Though cell polarization is implicit as the main premise in practically all the current models which have been proposed to explain the transepithelial active transport, the mechanisms involved in its subcellular and molecular development have not been elucidated. The aim of this research proposal is to understand the processes involved in the genesis and development of the epithelial cell polarization. The approach includes the use of cultured epithelial cell lines of renal origin that, like their "in vivo" counterpart, are able to perform transepthelial active transport. The development of epithelial cell polarization will be studied by monitoring different polarization markers during the conversion of a population of isolated cells into an epithelial membrane. In particular we intend to characterize the factors involved in the determination (signalling) of the polarization process by culturing the cells in interphases of different composition. We will determine which of the pathways proposed to explain the intracellular transport of membranes or macromolecules is or are involved in the assymetrical distribution of the membrane components. A combination of immunochemical, histochemical, and cell fractionation techniques will be used to approach this issue. Finally, we will analyze the mechanisms involved in the integration of polarization markers into the cell membrane. Different transport systems that become evident after their insertion in the cell membrane will be used in the analysis of this step.